(1) Field of the Invention
The present invention relates to radiation detectors used in the medical, industrial, nuclear and other fields in which radiations of high dose rates are measured.
(2) Description of the Related Art
Conventionally, this type of radiation detector includes a radiation detecting element formed of a compound semiconductor having large band gaps, such as GaAs, CdTe and HgI.sub.2. The radiation detecting element used in obtaining radiographic images, generally, includes a plurality of signal fetching electrodes formed on one surface of a compound semiconductor substrate and corresponding to respective pixels, and a common bias electrode formed on the other surface thereof.
When a reverse bias is applied to the bias electrode of such a radiation detecting element, and photons of a radiation such as X-rays impinge on the semiconductor, carriers (electrons and holes) corresponding in energy to the incident photons are formed in the semiconductor. Charge pulses are generated as a result of movement of these electrons and holes to the respective electrodes. The number of these charge pulses is counted for each pixel to obtain a radiographic image which is a count distribution image of the incident photons of the radiation.
With the compound semiconductor noted above, it is difficult to promote growth of high-resistance crystals for use as a radiation detecting element. Only crystals having numerous defects are available today.
These defects act as centers for trapping the carriers formed in the radiation detecting element by incident photons of a radiation. The defects having long time .tau..sub.D for which carriers remain trapped, reduce effective carrier mobility .mu..sub.r as expressed by equation (1) below, or act as scattering centers within the semiconductor crystals. This causes a reduction of output at a time of high dose radiation, as indicated by a curve (a) in FIG. 1. ##EQU1##
In the above equation, .mu..sub.0 is the intrinsic mobility of carriers, and .tau..sup.+ is a time taken before free carriers are trapped (i.e. trapping time).
In FIG. 1, reference (b) denotes a logic line disregarding double counting, and reference (c) a characteristic curve of pixels free from the output reduction noted above.
When output reductions take place as noted above, there will be two incident amounts of radiation for a certain count. This is detrimental to a correct detection of the incident amount of radiation. Further, the defects noted hereinbefore are distributed locally in the semiconductor crystals. Consequently, in the case of an array type radiation detecting apparatus for use in radiography, variations in detection sensitivity occur among the pixels, which produce an image having an irreparable irregularity.